The hippopotamus (Hippopotamus amphibius) is a popular zoo animal and an integral part of wetland ecosystems in its native continent of Africa. However, information about the reproductive physiology of the hippopotamus is limited compared to the other megaherbivores of Africa such as the elephant and the rhinoceros. This study is the first to report on the endocrine patterns associated with ovulation and pregnancy in the Nile hippopotamus. Fecal samples were collected 3 times per week from female hippopotami (N = 9) housed in captivity in Florida. Progesterone metabolites were extracted from the feces with 80% MeOH and quantified using a progestagen EIA. Fecal progestagen analysis indicated hippopotami had a cycle length of 35.3 +/- 1.3 days and ovulated throughout the year. Fecal progestagen concentrations during pregnancy (N = 11 pregnancies) were higher on average (833.5 +/- 115.2 ng/g; P < 0.05) than those observed during non-pregnant luteal phases (309.5 +/- 20.9 ng/g) and remained elevated throughout gestation. Fecal progestagen analysis revealed the onset of puberty in three females between 3 and 4 years of age and all conceived by their fourth cycle. Lactational suppression of ovulation during nursing was usually, but not always, observed and lasted for 33.9 +/- 1.8 weeks. The observed calving interval in females with lactational anovulation was 17 months. The majority of births (63.6%) occurring during the summer rainy season and the inconsistency of lactational anovulation suggested the possibility of environmental influences on reproduction.